Men in prison are five times more likely to commit suicide than those in the general population, a Lancet online study has found.

Mental illness and overcrowding are possible explanations, the University of Oxford researchers suggested.

The analysis of suicide rates in England and Wales from 1978 to 2003 found the difference was particularly striking in 15- to 17-year olds.

Prisoners in that age group were 18 times more likely to kill themselves.

A further increase in suicides may have to be expected if the policy of care for incarcerated individuals is not changed

Lancet editorial

The researchers found that 1,312 men aged 15 and over had committed suicide during the period studied.

Death rates were broken down into those occurring in males aged 15 to 17, 18 to 20, 21 to 24, 25 to 29, 30 to 39, 40 to 49, 50 to 59 and 60+.

They then looked at Office for National Statistics data on suicides for the same age groups.

The data showed the increased rate in male prisoners across the board, and especially in teenagers.

However, the researchers stress that the increased rate among 15- to 17-year-olds should be interpreted with caution, as it is based on just 28 deaths.

Detox treatment

But the team, led by Dr Seena Fazel, say their findings do show the extent of the problem of suicides in prison.

Dr Fazel told the BBC News website: "The incidence of suicide in prison had not really been reliably examined before."

He said he believed there were two main reasons for the increased number of suicides among prisoners, compared with men of the same age in the general population.

"There have been large numbers of people with mental illnesses who have left hospital because of the Care in the Community scheme.

"Half those who commit suicide in prison have a treatable mental illness."

Dr Fazel said over-crowding in prison was also a contributing factor, due to the stress it caused.

"The higher rate amongst teenagers might be caused by these boys having high rates of mental illness which are not being picked up, and their not receiving the detox they need for drug problems."

He said there were measures in place which were reducing suicide risk among prisoners.

Dr Fazel said the fact that the NHS was taking over the running of prison health services would help.

He added: "Other initiatives being implemented include removing prison bars. Ninety per cent of suicides involve people hanging themselves from the bars, so removing them is very helpful."

'Keep children safe'

A spokesman for the Sainsbury Centre for Mental Health, said: "We know that the 135,000 people who go into prison each year are at high risk of mental health problems.

"Investment in more and better prison health care is vital to ensure those who must be in prison get the care they need. Efforts are being made but it's an uphill task given the growing size of the problem.

"And people who should not be in prison because of their mental health problems must be found suitable alternatives."

Frances Crook, director of the Howard Centre for Penal Reform: "The findings regarding children are alarming. If the state is going to take the ultimate step, but taking away the child's freedom, it must keep them safe."

She said increased rates of mental illness among prisoners and over-crowding were factors in high suicide rates.

But she said prison itself could make someone feel suicidal.

The government has said that reducing the amount of self-harm among prisoners was a key priority for the Prison Service.